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Convection.
RELATIVE VALUE OF GOOD CONDUCTORS.

583

Substance.
	Relative Conducting Value.

Silver
	J OO

Cooler
	7V6

Brass
	2 7 "I

Iron
	*O  A II'O

Steel        ....... ..
	1 1 '6

Platinum
	8-4

Bismuth  ...
	1-8

Water      ............
	"147


	

Bad   conductors   are   of value   for   clothing   boilers,   steam
cylinders and pipes, &c.    (See p. 902.)

RELATIVE VALUE OF BAD CONDUCTORS (OBSTRUCTORS).

Substance.
	Relative Obstructing Value.

Silicate cotton or slag wool Hair felt ............ Cotton wool Sheep's wool      ......... Infusorial earth  ......... Charcoal ...
	100
 35H 82
 73'5 73*5
 71" &

Sawdust   ...        ...        .......
	6ri

Gasworks breeze ......... Wood, and air space
	43*4 357

Convection is a means of transmitting heat to liquids and
gases. A flask of water being placed over some heat source,
the lower or heated portion of the water becomes lighter and
rises to the surface, up the vertical centre-line, only to become
cool again and flow down the sides to the bottom. Thus are
continuous (convection' currents formed, which soon distribute
heat throughout the liquid. Similarly also is the air of a room